When you accept the trial version of Apple One Family plan you get an additional 200GB of storage. If you already have a 200GB plan the two parallel plans combine for the duration of the trail to become 400GB.
At the end of the trial you have two choices. If you decline to continue into the paid Apple One service you will loose the 200GB from Apple One and return to your 200GB iCloud storage plan. If you accept the paid Apple One subscription your iCloud storage plan billing will cease and you will go forward with a 200GB Apple One storage plan. Note that none of your iCloud content is affected; this is purely the administrative side of your storage plan.
Apple details all this in this support document.